A textural and chemical study of white mica in the South Mountain Batholith, Nova Scotia: primary versus secondary origin

Authors

  • L. J. Ham Nova Scotia Department of Mines and Energy, P.O. Box 1087, Halifax, Nova Scotla B3J 2X1
  • D. J. Kontak Nova Scotia Department of Mines and Energy, P.O. Box 1087, Halifax, Nova Scotla B3J 2X1

DOI:

https://doi.org/10.4138/1644

Abstract

White mica (HM) In peralumlnous granitoid rocks of the South Mountain Batholith (SMB) and East Kemptville leucogranite (EKL) of Nova Scotia have been examined to see if parameters can be used to distinguish between primary and secondary grains. Texturally very little of the UN in the SMB can unequivocally be classified as primary, whereas most of the UM In the EKL is consistent with such an origin. Although discriminant diagrams which utilize major element chemistry do not provide unambiguous divisions between primary and secondary HM, there appear to be some chemical trends which indicate that the bulk composition of the host rock is an important control. This is best exemplified by volatile (i.e. F) and trace element contents. Including the rare earth elements. For example, F, Li, Rb and Cs are systematically higher In UM from relatively more evolved units of the SMB. The importance of bulk rock composition is also indicated by the enrichment of lithophile elements In UM from the EKL. itself enriched in these same elements. The octahedral impurities in UM of the SMB are accommodated via blotltlc and pbengitic substitutions, whereas UM from the EKL is dominantly pbengitic. Comparison to experimentally determined stability fields for muscovite indicate that HM from the SMB re-equilibrated to 500-600oC In the more primitive units and 400-550oC In the more evolved units and greisens. Recent experimental data also suggest crystallization of the UM may have occurred at pressures of ca. 2 kb in melts with 2-3 wt.% H2O. RÉSUMÉ On a examiné les micas blancs (MB) dans les roches granltoldes péralumlneuses du Batholite de South Mountain (BSM) et du leucogranite d'East Kemptville (LEK) en Nouvelle-Écosse afin de déterminer si certains paramétres peuvent servlx a dlstinguer les grains primalres des grains secondaires. É l'égard de la texture, une très faible proportion des MB dans le BSM peuvent, sans l’ombre d'un doute, être classes comae primalres alors que la plupart des MB dans le LEK sont compatibles avec une telle orlglne. Bien que les diagrammes discriminant par chimle des éléments majeure ne procurent aucune Unite precise entre les MB primalres et secondaires, on semble y discemer des tendances chlmiques lndlquant une Influence préponderante de la composition totale de l'encaissant. Les contenus en éléments volatiles (i.e. F) et en traces, y comprises les terres rares, en sont la meilleure illustration. Par exemaple, la teneur en F, Li, Rb et Cs est systèmatiquement plus éleveé dans les MB provenant des unités plus évoluées, par comparaison, du BSM. L'importance de la composition totale de la roche est aussi attestée par l'enrichtssement en éléments lithophiles des MB du LEK (luimeme enrichl en ces éléments). L'accoramodation des impurités octaédriques dans les MB du BSM se fait par le biais de substitutions biotitiques et phengitiques alors que les MB du LEK sont surtout phengitiques. Une comparaison des MB du BSM aux doroaines de stabilites de la muscovite détermines experimentalement révèle que leur ré- équilibrage s'est effectue entre 500 et 600oC dans les unités les plus primitives et entre 400 et 550oC dans les unités plus évoluées et les greisens. À la lumlère de données expérimentales récentes, la cristallisation des MB aurait eu lieu àa des pressions d'environ 2 kb dans des bains ayant de 2 à 3 % en poids d'H2O. [Traduit par le journal]

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Published

1988-04-01

How to Cite

Ham, L. J., & Kontak, D. J. (1988). A textural and chemical study of white mica in the South Mountain Batholith, Nova Scotia: primary versus secondary origin. Atlantic Geoscience, 24(1). https://doi.org/10.4138/1644

Issue

Vol. 24 No. 1 (1988)

Section

Articles

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